Bis(allylic carbonate) monomers are frequently free-radically polymerized to yield hard polymers. Many of these monomers yield rigid polymers which exhibit high transparency to visible light when undyed, substantial hardness, and refractive indices that are sufficient for many, if not most, ophthalmic applications. For these reasons, such monomers find utility as precursors for optical lenses (especially ophthalmic lenses), lens blanks, sunglass lenses, face shields, filters, flat or curved sheets, coatings, and other optical elements.
One problem associated with the polymerization of bis(allylic carbonate) monomers is the relatively high shrinkage of the material which occurs during the course of polymerization. For example, shrinkage during the homopolymerization of diethylene glycol bis(allyl carbonate) monomer is approximately 12.5 percent. Such large degrees of shrinkage are especially troublesome where polymerizable bis(allylic carbonate) monomer compositions are polymerized in substantially enclosed molds typical of many casting operations. Although it is not desired to bound by any theory, it is believed that much, if not most, of the shrinkage can be attributed to the conversion of allylic groups to polymer segments.
As used herein, the term "shrinkage" is equal to (Dp-Dm)/Dp where Dp is the density of the final thermoset polymerizate at 25.degree. C. and Dm is the density of the casting composition at 25.degree. C. The term "percent shrinkage" is equal to shrinkage multiplied by one hundred.
One manner of reducing the degree of shrinkage during the casting operation has been to form a liquid prepolymer from bis(allylic carbonate) monomer, charge the mold with the prepolymer (and added initiator, when necessary), and polymerize the prepolymer to form a hard polymerizate. Inasmuch as a portion of the allylic groups have been converted prior to charging the mold, the shrinkage occurring during polymerization in the mold is reduced. The prepolymerization technique has therefore provided some success in dealing with the shrinkage problem.
The major obstacle to further reductions of shrinkage via the prepolymerization route has been gellation. Early prepolymerization techniques generally resulted in gellation when only a small proportion of the total available allyl groups had been utilized. For example, in forming prepolymers from diethylene glycol bis(allyl carbonate) monomer, gellation was typically observed after about 12 percent of the allylic groups had been utilized. Further work led to further improvements and prepolymers of diethylene glycol bis(allyl carbonate) having allylic utilizations of up to about 17 percent could be achieved prior to gellation; see, for example, Japanese Kokai Patent No. Sho 51[1976]-9188.
A major advance in the art of forming poly(alkylic carbonate)functional prepolymers has been described in detail in Application Serial No. 549,850, filed Nov. 9, 1983, now abandoned, and in copending Application Ser. No. 690,411, filed Jan. 10, 1985, now abandoned, the entire disclosures of which are incorporated herein by reference.
In accordance with a method of Application Ser. No. 549,850 and Application Ser. No. 690,411 poly(allylic carbonate)-functional monomer is dissolved in a solvent in which the polymer produced from such monomer is also soluble. Preferably, the initiator used to conduct the polymerization is also soluble in the solvent. The resulting liquid solution comprising poly(allylic carbonate)-functional monomer, solvent, and preferably initiator is then partially polymerized by heating the liquid solution to polymerization temperatures. The polymerization reaction is allowed to continue until more than 12 percent allylic utilization is attained. The degree of allylic utilization can be controlled by regulating the amount of initiator added to the liquid solution, the temperature at which the partial polymerization is performed, and the ratio of solvent to poly(allylic carbonate)-functional monomer. Generally, the greater the amount of initiator used, the higher is the allylic utilization. The higher the temperature of polymerization, the lower is the degree of allylic utilization. At constant temperature and employing a given amount of initiator, the higher the ratio of solvent to monomer, the lower is the degree of allylic utilization. Ordinarily however, if at constant temperature the ratio of solvent to monomer is increased and the amount of initiator employed is also sufficiently increased, the reaction can be driven to a higher degree of allylic utilization without the formation of gel than in a system containing less solvent.
In a preferred embodiment of Application Ser. No. 549,850 and Application Serial No. 690,411, from about 0.1 to about 1.5 weight percent of initiator, basis the amount of monomer, from about 0.5 to 5 milliliters of solvent per gram of monomer, and polymerization temperatures of from 28.degree. C. to about 100.degree. C. are used. The degree of allylic utilization can be monitored by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. The solvent in the resulting composition can be removed by known techniques, e.g., by evaporation or distillation, leaving a viscous liquid comprising a solution of poly(allylic carbonate)-functional prepolymer in poly(allylic carbonate)-functional monomer. This solution is typically a syrupy liquid having a kinematic viscosity (measured with a capillary viscometer) of from at least about 100 centistokes to about 100,000 centistokes, typically from about 1000 to 40,000 centistokes, more typically from about 500 to 2,000 centistokes, measured at 25.degree. C., and a bulk density at 25.degree. C. of from about 1.17 to about 1.23 grams per cubic centimeter. The solution is further characterized by having more than 12 percent allylic utilization, preferably from at least 15 to 50 percent allylic utilization, and, in a particularly preferred exemplification, from about 20 to 50 percent allylic utilization, as determined by infrared spectroscopy or nuclear magnetic resonance spectroscopy.
Application Ser. No. 549,850 and Application Ser. No. 690,411 indicate that the process therein described is applicable to poly(allylic carbonate)-functional monomers having an (allylic carbonate) functionality of from 2 to 5, preferably 2. Both aliphatic diol bis(allylic carbonate) monomers and bisphenol bis(allylic carbonate) monomers are discussed.
The most salient point in respect of Application Ser. No. 549,850 and Application Ser. No. 690,411 is that techniques are taught which permit the formation of poly(allylic carbonate)-functional prepolymers having up to 50 percent allylic utilization without gellation. This represents a major advance in the poly(allylic carbonate)-functional Prepolymer art.